1. Field of the Invention
The present invention relates to a noise reduction method and apparatus for reducing noise which overlaps detected signals in a common frequency band and is generated from sources different from a signal source during detection of distributed signals generated from the signal source at a plurality of measurement points.
2. Description of the Related Art
An example for such a detection is the Electrocardiogram (ECG), where the heart is the signal source and the distributed signals of body surface potentials generated from the heart are detected with multiple electrocardiographic electrodes mounted on limbs and the chest. A common problem in ECG detection is the muscle noise that arises from the skeletal muscle tremors and makes the detected ECG signals difficult to be analyzed by human or by an automatic analysis program. To remove the muscle noise, a low-pass filter, which generally has a cut-off frequency of 25 Hz, is commonly used at the present time. Since significant energy of muscle noise is typically at the frequency band higher than 25 Hz, this kind of filter can remove the muscle noise in some extents. However, since the spectrum of ECG signal is within the range of 0.05 to 100 Hz, this filter reduces high frequency components of the ECG waveform, causing distortion in ECG waveform, especially in the QRS peak magnitude. This disadvantage is undesirable for diagnosis since the QRS magnitude itself is often taken as a index in ECG analysis.
In recent years, signal-averaging technology is used to remove the muscle noise in ECG detection. Although this method is effective in some cases, it has limitations in application. First, this method is only applicable for periodic signals. Taken ECG as an example, the method can not be applied to ECG waveforms with premature ventricular contract (PVC) or other type of arrhythmias. Secondly, this method can not be implemented in real-time, which is desirable in many cases.
It is the fact that finding a method that can effectively reduce noise, which has a overlapping frequency band with detected signal, with no signal distortion is a difficult problem and therefore is still one of current topics in data processing.